Communication satellites are employed to receive electromagnetic signals from an earth station and then retransmit these signals to one or more earth stations. The signals contain information such as voice, video, and data for communication between the earth stations via the satellite. In essence, the purpose of a satellite is to transmit information from a sender to a receiver.
Typically, the power of the received signal at the satellite is weak because most of the power is lost through earth to satellite transmission path losses. The path losses are a result of the distance separating the satellite and the earth. The power of the received signal varies inversely as the square of the distance. For instance, the power of a signal transmitted by a feeder earth station may be around 1000 Watts, but the power of the signal received by the satellite may only be 1 nano Watt (10.sup.-9 W).
Because the power of the signal received by the satellite is too weak for transmission, the satellite has an amplifier to amplify the received signal. After amplification, the satellite transmits the amplified signal back to a receiving earth station. The satellite may employ additional techniques such as demodulation and modulation to process the received signal before transmission. Again, during transmission, most of the power of the transmitted signal is lost through satellite to earth transmission path losses. For instance, the satellite may transmit a signal having a power of 10 Watts after amplification, but only 10 pico Watts (10.sup.-12 W) is received by the feeder earth station.
Satellites employ antennas to transmit and receive signals because antennas have the ability to direct the signals to a specific location and the ability to tune to signals emanating from a specific location. Antennas can transmit signals having given frequencies to a specific location by focusing the signals into what is referred to as a radiation pattern. Similarly, antennas tune to the same radiation pattern to receive signals with the given frequencies emanating from the specific location. Antennas have the property of transmitting and receiving identical radiation patterns because they are reciprocal devices. Typically, antennas perform both of these operations at once by using slightly different signal frequencies in a frequency band. However, the variation of the frequencies are usually of the same magnitude so that the radiation pattern is the same in both modes.
In the transmit mode, the antenna forms a radiation pattern by increasing the power transmitted in a selected direction while reducing the power transmitted in other directions. The measure of the ability of an antenna to transmit power in a selected direction rather than equally in all directions is referred to as the directivity of the antenna. An interrelated concept to directivity is gain. The gain of an antenna is the measure of the ability of an antenna to increase the power to a given area by reducing the power to other areas.
In the receive mode, the antenna gathers energy from impinging electromagnetic energy. Because of reciprocity, the antenna is tuned to gather energy emanating from areas within the radiation pattern while being non-receptive to signals emanating from all other areas. The measure of the ability of an antenna to gather energy from a specific area is referred to as the effective aperture of the antenna. In general, a high effective aperture antenna in the receive mode also exhibits a high gain in the transmit mode.
Typically, satellites employ some sort of antenna assembly. The antenna assembly consists of a main reflector and a feed assembly. The main reflector is usually a parabolic reflector or a shaped reflector. In the transmit mode, the feed assembly illuminates the main reflector with an electromagnetic energy beam. The main reflector then reflects and focuses the electromagnetic energy beam into a radiation pattern for transmission to earth. In the receive mode, the main reflector focuses impinging electromagnetic energy from a radiation pattern into a reflected beam on the feed assembly.
The feed assembly is usually located at a focal point of the main reflector either on the axis perpendicular with the center of the main reflector or offset from this axis. Because the feed assembly may intercept a small part of the reflected beam from the main reflector, the feed assembly is often offset so that it is outside of the reflected beam. This is especially true for main reflectors having a small size.
The feed assembly may have various configurations. For instance, the feed assembly may consist of a single feed element such as a feed horn directed towards the main reflector. The feed assembly may also consist of a sub-reflector directed at the main reflector and a feed element directed at the sub-reflector. In this scenario, the feed element illuminates the sub-reflector with electromagnetic energy. The sub-reflector then reflects this energy to illuminate the main reflector.
Because of the extreme losses caused by the transmission distance, it is desirable to reduce the amount of wasted power transmitted from the satellite antenna. Power is wasted when unwanted areas such as the ocean receive a portion of the transmitted signal. Accordingly, the antennas are designed to transmit signals having radiation patterns such that the pattern contour fits the shape of a desired coverage region. For instance, the desired coverage region may be the island of Japan, the continental United States, or even a time zone.
Similarly, because of the transmission losses, it is desirable for the antenna to tune to the desired coverage region so that it gathers as much power as possible from the region while not gathering power from outside of the region. As discussed above, when an antenna is designed to transmit energy to a desired coverage region, because of reciprocity, this region is also where the antenna tunes to gather energy.
One known method for producing shaped contour radiation patterns is an array-fed parabolic reflector. Another known method is a direct radiating planar array. Both approaches generally employ passive beamforming networks to weight the array elements. However, there are several disadvantages associated with these methods. First, they need operating power which is a problem for a satellite that has limited supply power available. Second, they are expensive to incorporate in a satellite. Third, the electromagnetic energy loss associated with the passive beamforming networks may be intolerable.
Another known method for producing shaped contour radiation patterns is to use a feed assembly with a shaped main reflector. The shaped main reflector is a main reflector that has had its surface shaped to produce a desired radiation pattern. A primary disadvantage associated with shaped reflectors is that the radiation patterns generated by these reflectors are fixed and have to be decided upon before launch of the satellite. Specifically, the shape of the reflector and the position of the feed are designed for a given fixed radiation pattern and position of the satellite. Because of the expanding satellite market, the requirements are continuously changing requiring on-orbit reconfigurability, i.e., changing the radiation patterns while in orbit.
In addition to using the previously introduced beamforming networks to change the radiation pattern of a shaped reflector, prior designs discuss changing the surface of the shaped reflector while in orbit. This is a fairly complex scenario requiring a number of actuators located at many points over the reflector surface. No practical implementation has been accomplished for a satellite in orbit due to the complexity.